Introduction:High-resolution CT (HRCT) has assumed an important role in the assessment and evaluation of patients with suspected chronic infiltrative lung disease, limiting the diagnostic possibilities, directing further evaluation, and in some cases, obviating biopsy. This study retrospectively assesses the accuracy of HRCT in distinguishing three common infiltrative lung diseases: idiopathic pulmonary fibrosis (IPF), hypersensitivity pneumonitis (HP), and nonspecific interstitial pneumonia (NSIP).

Two independent readers assessed the HRCT images, made a first-choice diagnosis, and noted their degree of confidence in the diagnosis

A large number of HRCT findings and their extent were independently assessed as to their diagnostic value

A general linear model was used to identify HRCT features that independently differentiated IPF, chronic HP, and NSIP

Weighted kappa statistic was used to assess interobserver agreement

Results:A confident diagnosis was made in 70 (53%) of 132 readings. This diagnosis was correct in 66 (94%) of 70 readings. The accuracy for the entire cohort was 80%. Interobserver agreement for confident diagnosis was good to excellent (kappa 0.77–0.96).

The HRCT features that best differentiated IPF from the other diseases were a basal predominance of honeycombing, the absence of relative subpleural sparing, and the absence of centrilobular nodules (P ≤ 0.004)

The HRCT features that best distinguished chronic HP were lobular areas with decreased attenuation and vascularity (ie, mosaic perfusion) in five or more secondary lobules in more than four lobes, the lingula being considered a separate lobe; centrilobular nodules; and absence of a lower zone predominance of abnormalities (P ≤ 0.008)

The HRCT features that best distinguished NSIP were relative subpleural sparing, absence of lobular areas of decreased attenuation as defined above, and lack of honeycombing (P ≤ 0.002)

Expert Opinion:This paper is most helpful in the identification of specific HRCT findings, and combinations of findings, that are useful in distinguishing among these three common infiltrative lung diseases. Although a number of HRCT findings have been previously reported as typical of IPF, chronic HP, and NSIP, the value of these specific findings in making an accurate diagnosis had not been assessed. Recognizing these findings will enable a more accurate and confident HRCT interpretation.

The study is limited in that it considers only three possible diseases, and this may have resulted in a higher accuracy and interobserver agreement than otherwise would be the case. But as the authors point out, the selection of patients was intentional, owing to the considerable overlap of the clinical, functional, and radiologic manifestations of IPF, chronic HP, and NSIP.

On the other hand, since the authors required a histologic diagnosis for inclusion in the study, there may have been a bias toward cases without typical HRCT findings or without obvious clinical associations. In practice, biopsy may not be obtained in patients with typical HRCT findings of IPF, in patients with typical findings of chronic HP and a history of exposure, or in patients with typical NSIP who have collagen-vascular disease. Despite this possibility, a high accuracy in HRCT diagnosis was reported.